U.S. patent number 5,614,579 [Application Number 08/352,938] was granted by the patent office on 1997-03-25 for process for the preparation of tapered copolymers via in situ dispersion.
This patent grant is currently assigned to Bridgestone Corporation. Invention is credited to James E. Hall, David M. Roggeman.
United States Patent |
5,614,579 |
Roggeman , et al. |
March 25, 1997 |
Process for the preparation of tapered copolymers via in situ
dispersion
Abstract
A process is provided for the preparation of tapered copolymers
by the nonaqueous dispersion polymerization of a mixture of 30 to
70% by weight of a conjugated diolefin monomer, preferably
butadiene, and 30 to 70% by weight of a vinyl substituted aromatic
monomer, preferably styrene, in a liquid hydrocarbon dispersion
medium with an anionic initiator catalyst system in the presence of
a block copolymeric dispersing agent. At least one block of the
dispersing agent is prepared prior to the dispersion polymerization
reaction and at least one block of the dispersing agent is prepared
in situ during the dispersion copolymerization. The block of the
dispersing agent that is prepared in situ has the polymer structure
of the tapered copolymer.
Inventors: |
Roggeman; David M. (North
Royalton, OH), Hall; James E. (Mogadore, OH) |
Assignee: |
Bridgestone Corporation (Tokyo,
JP)
|
Family
ID: |
26835153 |
Appl.
No.: |
08/352,938 |
Filed: |
December 9, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
137332 |
Oct 18, 1993 |
|
|
|
|
995118 |
Dec 22, 1992 |
5331035 |
|
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Current U.S.
Class: |
524/457; 524/461;
525/250; 525/271; 525/315; 525/316; 525/514; 525/89; 526/201 |
Current CPC
Class: |
C08F
297/04 (20130101) |
Current International
Class: |
C08F
297/00 (20060101); C08F 297/04 (20060101); C08K
003/02 () |
Field of
Search: |
;524/457,461
;525/89,250,271,314,315,316 ;526/201 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Mulcahy; Peter D.
Attorney, Agent or Firm: Troy, Sr.; Frank J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of copending application
Ser. No. 08/137,332, filed Oct. 18, 1993 which is a
continuation-in-part of Ser. No. 07/995,118, filed Dec. 22, 1992,
now U.S. Pat. No. 5,331,035.
Claims
We claim:
1. A process for the preparation of a tapered copolymer having no
sharply defined glass transition temperature and having 30 to 70%
by weight of vinyl aromatic contributed units and 30 to 70% by
weight of diene monomer contributed units in a hydrocarbon
dispersing medium comprising the following steps:
(1) injecting into a reactor a charge of 75 to 100% by weight of a
diene monomer, 0 to 25% by weight of a vinyl aromatic hydrocarbon
monomer and a catalytically effective amount of an anionic
initiator in a hydrocarbon dispersing medium;
(2) polymerizing the charge of step (1) to form a preformed block
of a dispersing agent, the preformed block being soluble in the
hydrocarbon dispersing medium;
(3) injecting into the reactor a charge of:
(a) a vinyl aromatic hydrocarbon monomer,
(b) a conjugated diene monomer, and
(c) an additional charge of an anionic initiator
wherein during the course of the dispersion polymerization process
the percent by weight of the diene monomer being charged into the
reactor is (100-X) %, wherein X% represents the weight percent of
the vinyl aromatic hydrocarbon monomer being charged into the
reactor and wherein X % incrementally varies between a maximum
charge between about 100 to 50% and a minimum charge between about
0 to 30% to produce:
(I) a block copolymer dispersing agent consisting of the preformed
block and a block formed in situ during step (3) and
(ii) a tapered copolymer having no sharply defined glass transition
temperature and having vinyl aromatic hydrocarbon monomer
contributed units ranging from about 30 to 70% by weight.
2. The process as defined in claim 1 wherein the conjugated diene
monomer is 1,3-butadiene.
3. The process as defined in claim 1 wherein the vinyl aromatic
monomer is styrene.
4. The process as defined in claim 1 wherein the hydrocarbon
dispersing medium comprises hexane.
5. The process as defined in claim 1 wherein the hydrocarbon
dispersing medium comprises at least 60% hexane.
Description
FIELD OF THE INVENTION
The present invention relates to an anionic tapered
styrene-butadiene type rubber polymerization process conducted in a
nonaqueous dispersion utilizing butadiene-type and styrene-type
monomers and a dispersing agent formed in situ during
polymerization.
BACKGROUND OF THE INVENTION
In many prior art nonaqueous dispersion polymerization systems,
organic dispersing medium has been utilized having poor solvent
properties for the polymer being produced. A dispersing agent was
therefore utilized in the organic medium in order to disperse the
polymer being formed throughout the medium. These dispersing agents
or dispersants were generally polymeric materials such as block
copolymers, random copolymers, or homopolymers as described in U.S.
Pat. Nos. 4,098,980 and 4,452,960.
Styrene-butadiene rubbers (SBR) have generally been prepared in
solvents in which SBR is soluble, however, only SBR's having a
styrene content of less than 35% are soluble in hexane or other
aliphatic solvents. These higher styrene content SBR polymers are
not completely insoluble in the aliphatic solvents, and, in fact,
are highly swollen in these solvents. However, SBR's having a
styrene content greater than 35% necessarily have been polymerized
in aromatic or cycloaliphatic solvents via solution
polymerization.
The applicant first determined that certain pre-made dispersing
agents can be utilized to conduct the nonaqueous dispersion
polymerization production of SBR having a styrene content greater
than 35% by weight in aliphatic dispersing medium such as hexane.
In these first dispersion SBR studies, a single diblock polymer
consisting of a short block (5-10% of total polymer) of hexane
soluble polybutadiene and a long block (90-95% of total) of high
styrene content SBR was prepared in the absence of a dispersing
agent. Synthesis of this polymer structure in hexane resulted in
either extremely viscous cements or the very undesirable phase
separation. Although the dispersion process using a pre-made
dispersant works well, it has one shortcoming from a practical or
commercial scale up point of view. The dispersant must be prepared
separately and stored for subsequent use in the polymerization
process. Storage tank and transfer lines require a large capital
expenditure and the synthesis of the dispersant and transfer time
into the polymerization reactor results in higher production costs.
It is therefore desirable to provide a dispersion polymerization
process in which there is no need to store the dispersing agent
prior to the commencement of the dispersion polymerization
process.
It is an object of the present invention to provide a
polymerization process to produce tapered copolymers of styrene and
butadiene monomers in a nonaqueous dispersion into tapered
copolymers having 30 to 70% by weight of vinyl aromatic hydrocarbon
such as styrene in the presence of a dispersing agent prepared in
situ, that is, during the polymerization reaction.
Such a nonaqueous dispersion polymerization process offers many
advantages including improved stable dispersions, improved heat
transfer, energy savings, high polymer concentrations in the
reaction medium, increased production capacity, and the production
of very high molecular weight tapered copolymers; and no need to
store the dispersing agent prior to its use.
It is a further embodiment of the present invention that a stable
dispersion of SBR can be made at styrene levels of 30 to 45% in the
SBR in solvents of higher solubility using the in situ dispersion
tapered copolymer process as compared to the in situ dispersion
linear, random SBR copolymer process as shown in U.S. Pat. No.
5,331,035. Normally SBR having between a 30 to 35% styrene content
is soluble in hexane. This new process extends the advantages of
dispersion polymerization to a lower, unexpected styrene level.
SUMMARY OF THE INVENTION
In accordance with the present invention, a process is provided for
the preparation of tapered copolymers by the nonaqueous dispersion
polymerization of a mixture of 30 to 70% by weight of a conjugated
diolefin monomer, preferably butadiene, and 30 to 70% by weight of
a vinyl substituted aromatic monomer, preferably styrene, in a
liquid hydrocarbon dispersion medium with an anionic initiator
catalyst system in the presence of a block copolymeric dispersing
agent. At least one block of the dispersing agent is prepared prior
to the dispersion polymerization reaction and at least one block of
the dispersing agent is prepared in situ during the dispersion
copolymerization. The block of the dispersing agent that is
prepared in situ has the polymer structure of the tapered
copolymer.
DETAILED DESCRIPTION OF THE INVENTION
The copolymer rubbers prepared by the process of the instant
invention are tapered copolymers formed by the copolymerization of
a conjugated diene monomer and a vinyl substituted aromatic
monomer.
The conjugated diene monomers utilized in the synthesis of such
tapered copolymer rubbers generally contain from 4 to 12 carbon
atoms. Diene monomers containing from 4 to 8 carbon atoms are
generally preferred for commercial processes. For similar reasons,
1,3-butadiene and isoprene are the most commonly utilized
conjugated diolefin monomers. Some additional conjugated diolefin
monomers that can be utilized include 2,3-dimethyl-1,3-butadiene,
piperylene, 3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, and the
like, alone or in admixture.
Vinyl substituted aromatic monomers, also referred to as vinyl
aromatic monomers (VAM) suitable for use in preparing the tapered
copolymers of this invention include any vinyl or alpha-methyl
vinyl aromatic compounds capable of being polymerized by an anionic
initiator. Particularly useful monomers for this purpose are vinyl
aryl and alpha-methylvinyl aryl compounds such as styrene,
alpha-methylstyrene, vinyl toluene, vinyl naphthalene,
alpha-methylvinyl toluene, vinyl diphenyl, and corresponding
compounds in which the aromatic nucleus may have other alkyl
derivatives up to a total of 8 carbon atoms. Certain vinyl
substituted aromatic monomers are not suitable for use in this
dispersion polymerization process because homopolymers of these
monomers are soluble in linear alkane solvents such as hexane and
their copolymers with diene are also soluble. A specific example of
an unsuitable monomer type is tert-butyl styrene.
The preferred comonomers for use in the process of the present
invention are styrene and butadiene for production of a tapered SBR
copolymer. In the production of the tapered copolymers of the
present invention, the vinyl substituted aromatic monomer
contributed content ranges from 30 to 70% by weight; preferably 30
to 60% by weight and most preferably, 30 to 45% by weight, and the
diene monomer contributed content ranges from 30 to 70% by weight,
preferably 40 to 70% by weight, and most preferably, 55 to 70% by
weight.
The tapered copolymers produced by the process of the present
invention can be prepared from any combination of each of the
aforementioned conjugated diene and vinyl aromatic monomers. While
the following discussion relates to the production of tapered
styrene-butadiene rubbers (SBR) from styrene and butadiene
monomers, it is apparent that this discussion encompasses the use
of any combination of the above-identified vinyl-substituted
aromatic hydrocarbons and conjugated dienes. The SBR-type
copolymers prepared by the process of the present invention have an
average molecular weight of 0,000 to 2,500,000 preferably 75,000 to
500,000. In addition to the ability to make high molecular weight
polymers possessing good hot tensile strength, these copolymers
have good oil acceptance or extendibility, modulus, tensile
strength and stability against heat and aging.
The solvents, also known as the dispersing medium, used in the
present polymerization process are aliphatic hydrocarbons, such as
cycloaliphatic, branched and linear aliphatic hydrocarbons,
including butane, pentane, hexane, heptane, isopentane, octane,
isooctane, nonane, cyclohexane, isohexane and the like and mixtures
thereof. Solvents are employed within such a range as being
necessary to maintain a dispersion state in said solvent and for
properly controlling stability of a polymer dispersion. The
insolubility of SBR in a solvent is a function of molecular weight
of the polymer, temperature, and the solubility parameter, which is
the square root of the cohesive energy density, that is; ##EQU1##
wherein E is internal energy and V is the molar volume. For
polymers, it is often best to calculate s.p. as displayed in the
article "A Method for Estimating the Solubility Parameters and
Molar Volumes of Liquids" in Polymer Engineering & Science,
Vol. 14, No. 2, pp 147-154 (1974). The calculated s.p. is 8.6 for
polybutadiene, 9.17 for a tapered SBR having
a 30% styrene content, and 10.5 for polystyrene. The s.p. of
n-hexane is 7.28 and a 30% styrene tapered SBR has only partial
solubility in n-hexane. The solubility parameter (s.p.) of SBR or
other random copolymer produced by the present invention is
preferably at least about 1.9 greater than the s.p. of the solvent
or dispersing medium, so that the tapered SBR is not completely
soluble in the dispersing medium, thereby forming an acceptable
dispersion.
The preferred solvent for use as a dispersing medium in the present
process is n-hexane. While the solvent may consist of up to 100% of
non-cyclic aliphatic hydrocarbons, preferably up to 70% of
non-cyclic aliphatic hydrocarbons, up to 40% by weight of the total
solvent can be provided by alicyclic hydrocarbons such as
cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane
and aromatic hydrocarbons such as benzene and toluene, or mixtures
thereof. A higher percentage of VAM units in the tapered SBR allows
for a higher percentage of alicyclic and aromatic hydrocarbons to
be present in a solvent mixture. However, for a tapered SBR with
approximately less than 45% styrene content, no more than 20% of
the solvent should consist of one or more alicyclic hydrocarbons
such as cyclohexane. The tapered copolymer product preferably
contains 10 to 50% weight of solids relative to the liquid
hydrocarbon dispersing medium to yield a fluid polymer dispersion
that can be easily handled.
The copolymerization process of the present invention is performed
in a nonaqueous dispersing medium in the presence of an anionic
initiator catalyst system and a block copolymer dispersing agent
that is prepared in situ during the copolymerization process. The
block copolymer dispersing agents useful in the present invention
are polyblock copolymers, in that they are selected from a variety
of polymers containing at least two blocks linked by chemical
valences wherein at least one of said blocks ("A" block) is soluble
in the dispersion medium and at least another of said blocks ("B"
block) is insoluble in the dispersion medium. The dispersing agent
acts to disperse tapered copolymers hereinafter identified as `C`
copolymers, formed from conjugated dienes and vinyl aromatic
monomers which are formed in the presence of the dispersing agent.
The insoluble "B" block provides an anchor segment for attachment
to the `C` tapered copolymer, i.e. the tapered SBR polymer. The
soluble "A" block of the dispersing agent provides a sheath around
the otherwise insoluble copolymer and maintains the copolymeric
product as numerous small discrete particles rather than an
agglomerated or highly coalesced mass. The insoluble "B" block may,
if desired, contain a plurality of pendent groups.
The dispersing agent of the present invention can be represented by
the formula:
and the tapered copolymer is represented by "C"; wherein "A" is a
hydrocarbon soluble block formed by the polymerization of 75 to
100% by weight of diene monomer and 0 to 25% by weight of vinyl
aromatic hydrocarbon monomer, and "B" and "C" are identical
representing tapered block copolymers having 30 to 70% by weight of
vinyl aromatic hydrocarbon monomer contributed units and 30 to 70%
by weight of diene monomer contributed units. Tapered blocks
containing as low as 30% styrene content are dispersed in
hydrocarbon solvents when made by the in situ dispersion taper
process. The dispersed tapered block "B" and the tapered copolymers
"C" are simultaneously formed during a semi-batch polymerization.
In a semi-batch polymerization process, the monomers are metered
into the reactor containing the "A" block and anionic
initiator.
The soluble "A" block of the dispersing agent comprises about 1 to
about 15% by weight of the total dispersion copolymer including the
dispersing agent and the "C" tapered copolymer, i.e., the SBR-type
tapered copolymer. The insoluble "B" block of the dispersing agent
is prepared in situ during the polymerization of the SBR-type
tapered copolymer, therefore the "B" block has the same composition
as the "C" tapered copolymers, namely the SBR-type tapered
copolymer formed during the dispersion copolymerization process.
The total dispersion copolymer composition preferably contains
about 2 to about 10% by weight of the soluble "A" block and about
90 to about 98% by weight of the insoluble "B" block and "C"
copolymers, most preferably from 4 to 8% by weight of "A" and 92 to
about 96% by weight of total "B" block and "C" copolymers. The
number average molecular weights M.sub.n of each "A" block is
preferably at least 500 and a maximum of 200,000, most preferably
1,000 to 50,000.
The number average molecular weights of each "B" block is the same
as the `C` copolymers or SBR-type random polymer, namely at least
20,000 and a maximum of 2,500,000, preferably 75,000 to
500,000.
While it is believed that the soluble "A" can be prepared from any
monomer providing a soluble block in the dispersing medium subject
to known anionic polymerization constraints, it is preferred that
the soluble "A" block be selected from a polymer formed by
polymerization of conjugated diene monomers or be selected from a
copolymer formed by copolymerization of conjugated diene monomers
and vinyl substituted aromatic monomers. The soluble "A" block is
most preferably selected from a polymer or a copolymer formed from
75 to 100 parts by weight, preferably 75 to 98 parts, of conjugated
diene monomer contributed units and 0 to 25 parts by weight,
preferably 2 to 25 parts, of vinyl substituted aromatic monomer
contributed units with all polymer or copolymer blocks being
soluble in the hydrocarbon dispersion medium.
The insoluble "B" block is produced in the dispersion
polymerization process during the formation of the tapered
copolymer having the same composition as the tapered copolymer. The
insoluble "B" block is anchored to the surface of or the outer
layer of the copolymer particle by physical adsorption processes,
as for example, by van der Waals forces. Therefore, its main
criteria for success as an anchor is to be relatively immiscible in
the dispersing medium. The "B" block can be prepared by the
copolymerization of 30 to 70 parts by weight of conjugated diene
monomer contributed units and 30 to 70 parts by weight of vinyl
substituted aromatic monomer contributed units.
Diblock (A-B) dispersing agents are typically prepared utilizing
monolithium anionic initiators. The use of dilithium anionic
initiators promotes the production of triblock B-A-B dispersing
agents. The dispersing agents prepared in situ and used in the
preparation of the tapered SBR copolymers are recovered as a blend
with the "C" copolymers, i.e. tapered SBR copolymers. The
dispersing agents are prepared and present in an amount ranging
from about 2 to 50%, preferably 5 to 35%, and most preferably 10 to
25% by weight of the total weight of the dispersion copolymer which
includes the dispersing agent and the subsequently formed "C"
tapered copolymer, i.e. tapered SBR copolymer.
The catalyst systems for use in preparing the tapered SBR
copolymers and the dispersing agent are anionic initiators,
preferably any organolithium catalyst known in the art as being
useful in the polymerization of vinyl aromatic hydrocarbons and
conjugated dienes. Suitable catalysts which initiate polymerization
of the monomer system and dispersing agent include organolithium
catalysts which have the formula R(Li).sub.x wherein R represents a
hydrocarbyl radical of 1 to 20, preferably 2-8, carbon atoms per R
group, and x is an integer of 1-4, preferably 1 or 2. Typical R
groups include aliphatic radicals and cycloaliphatic radicals, such
as alkyl, cycloalkyl, cycloalkylalkyl, alkylcycloalkyl, aryl and
alkylaryl radicals.
Specific examples of R groups for substitution in the above formula
include primary, secondary and tertiary groups such as methyl,
ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, n-amyl,
isoamyl, n-hexyl n-octyl, n-decyl, cyclopentyl-methyl,
cyclohexyl-ethyl, cyclopentyl-ethyl, methyl-cyclopentylethyl,
cyclopentyl, cyclohexyl, 2,2,1-bicycloheptyl, methylcyclopentyl,
dimethylcyclopentyl, ethylcyclopentyl, methylcyclohexyl,
dimethylcyclohexyl, ethylcyclohexyl, isopropylcyclohexyl, and the
like.
Specific examples of other suitable lithium catalysts include:
phenyllithium, naphthyllithium, 4-butylphenyllithium,
p-tolyllithium, 4-phenylbutyllithium; 4-butylcyclohexyllithium,
4-cyclohexylbutyllithium, 1,4-dilithiobutane, 1,10-dilithiodecane,
1,20-dilithioeicosane, 1,4-dilithiobenzene,
1,4-dilithionaphthalene, 1,10-dilithioanthracene,
1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,
1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,
1,3,5,8-tetralithiodecane, 1,5, 10,20-tetralithioeicosane,
1,2,4,6-tetralithiocyclohexane, 4,4'-dilithiobiphenyl, and the
like.
Mixtures of different lithium catalysts can also be employed,
preferably containing one or more lithium compounds such as
R(Li).sub.x. The preferred lithium catalyst for use in the present
invention is n-butyllithium.
Other lithium catalysts which can be employed are lithium dialkyl
amines, lithium dialkyl phosphines, lithium alkyl aryl phosphines
lithium diaryl phosphines and trialkyl tin lithium such as
tributyl-tin-lithium.
Anionic initiators are typically employed in catalytically
effective amounts ranging from 0.2 millimoles to 100 millimoles of
anionic initiator per hundred grams of monomer in the reaction
vessel. Approximately 1 to 10 mmole, preferably 0.2 to 5.0 mmole of
anionic initiator per hundred grams of monomer is preferred for use
in the present process.
All amounts of anionic initiator are indicated by hundred grams of
monomer or by molar ratios of components in the instant invention
and are considered to be catalytically effective amounts, that is,
effective amounts for initiating and conducting polymerization of
the dispersing agent and the disclosed monomer systems to produce
copolymers of the present invention.
It is preferable to use 10 to 50% by weight of the total anionic
initiator employed in the process to prepare the initial "A" block
of the dispersing agent. The remaining portion of the initiator is
then added to the reactor prior to sequential charging of the
monomers to simultaneously produce both the "B" tapered block of
the dispersing agent and the tapered copolymer from vinyl aromatic
monomers and conjugated diene monomers.
Modifying agents such as ethers, tertiary amines, chelating ethers
or amines, and sodium or potassium alkoxides or alkyls, may be
added to increase the 1,2-addition reaction of the diene monomer in
the SBR. Such modifying agents are well known in the art, such as
tetrahydrofuran, tetramethylethylene diamine, diethylether, bis
oxolanyl propane (OOPS), and the like, and these modifying agents
may be employed in amounts generally ranging from 1:10 to 100:1
molar ratio of the modifier to anionic initiator. The 1,2-addition
product can be increased from the 5 to 15% range to as high as 90%
of the diene monomer units being incorporated into the "A" or "B"
block of the dispersing agent and the "C" copolymer.
The preferred 1,2-vinyl content of the "B" tapered block and the
"C" tapered copolymer, i.e. SBR produced in accordance with the
process of the instant invention, ranges between 15 to 65% of the
diene monomer contributed units. The 1,2-vinyl content in the diene
contributed units of the "B" tapered block of the dispersing agent
is identical to the desired final 1,2-vinyl content of the "C"
tapered copolymer being produced herein.
The concentration of butadiene and styrene monomers utilized in
such a nonaqueous dispersion polymerization reaction mixture can be
varied from about 10 to about 50% by weight of monomers based upon
the total reaction mixture. It is preferred to have a final polymer
concentration ranging from 20 to 35% by weight based upon the total
reaction mixture.
It is desirable to conduct this polymerization in an oxygen-free
and moisture-free environment. For example, it is desirable to
sparge the reaction mixture with dry nitrogen and to run the
polymerization under a dry nitrogen atmosphere. The pressure in the
reaction system during the polymerization generally is a function
of the polymerization temperature, the monomer concentration, and
the boiling point of nonaqueous dispersion medium. The
polymerization pressure is preferably maintained within the range
between 1.0 and 15 atmospheres.
The nonaqueous dispersion polymerization is conducted in a
semi-batch process. As polymerization in the reactor is exothermic
the reaction is typically conducted at 150.degree. to 350.degree. ,
preferably 200.degree. to 300.degree. F. with a residence time in
the reactor of 0.3 to 6.0 hours preferably about 0.5-2.0 hours. The
monomers are metered into the reactor containing the dispersion
medium and an anionic initiator. The "A" block of the dispersing
agent can either be metered into the reactor with the monomers or
added to the reactor before the monomers are added or is preferably
pre-made in the reactor prior to the metered addition of the
monomers for the preparation of the tapered "B" block and "C"
copolymer.
During the preparation of the taper "B" block and "C" copolymer,
the monomer charge rate into the reactor is preferably constant.
Tapering is controlled by changing the ratio of the diene and vinyl
aromatic monomer charge rates. The percentage by weight of the
vinyl aromatic monomer (VAM) in the overall monomer charge
incrementally decreases from a maximum amount in a maximum charge
range between about 50 to 100% by weight of the current total
reactor monomer charge to a minimum amount in a minimum charge
range between about of 0 to 30% by weight. The corresponding
percentage of diene monomer by weight in the initial overall
monomer charge during the charge cycle incrementally inversely
changes from an initial charge ranging from 0 to 50% by weight of
the overall monomer charge to a maximum charge ranging from 70 to
100% by weight. The overall average vinyl aromatic content of the
tapered "B" block and tapered copolymers "C" produced according to
the present invention is in the range of 30 to 70% by weight of the
total monomer charge.
For purposes of this invention monomer charge is defined as the
amount of monomer flowing into the reactor at a specific point in
time. Thus, if X% represents the amount by weight percent of vinyl
aromatic monomer, hereinafter discussed as styrene, of the total
monomer charge being charged into the reactor, then (100-X)%
represents the amount by weight percent of diene monomer charge,
hereinafter discussed as butadiene, continuously being charged into
the reactor in the formation of tapered styrene-butadiene block
copolymers.
In a preferred embodiment of the present invention, the value of X
% changes from a maximum charge between 60 to 100% to a minimum
charge between 0 to 15%. By varying the weight ratio of butadiene
to styrene in the reactor charge, the process of the present
invention produces tapered styrene-butadiene copolymer so that the
final recovered tapered styrene-butadiene copolymer contains 30 to
70% by weight of styrene or other vinyl aromatic monomer and
possesses varying chain stiffness along the length of the polymer
molecule and is suitable as an ideal rubber tread composition.
The present process incrementally changes the vinyl aromatic
monomer to diene monomer feed ratio in the reactor while the
modifier to initiator feed ratio in the reactor is either
maintained as constant or is also adjusted during the course of the
polymerization. The combination of polymerization temperature, flow
rate, changing or constant modifier concentration, and changing
vinyl aromatic monomer feed to diene monomer ratio results in a
tapering styrene-butadiene copolymer.
The feed ratio of modifier to anionic initiator in the reactor can
be incrementally increased to provide an increasing
1,2-microstructure percentage in diene contributed units along the
backbone chain during the polymerization preferably in a molar
ratio ranging between 0 and 5 moles of chelating modifier per mole
of anionic initiator and between 0 to 400 moles of non-chelating
modifier per mole of anionic initiator.
Process conditions such as the initial and maximum temperature of
the polymerization reaction can independently affect the final
1,2-microstructure content of the 1,3-diene copolymers or polymers.
These conditions can be controlled for each monomer reaction system
to produce the final desired average 1,2-microstructure content of
from about 10 to 90%. It is desirable to produce polymers and
copolymers having an average 1,2-microstructure between 20 and 35%
in the 1,3-diene monomer contributed units. In the production of
tapered copolymers having a 30 to 40% by weight of vinyl aromatic
monomer contributed units, the 1,2-microstructure content in the
diene contributed units is preferably less than 25%, while tapered
copolymers having higher percentages vinyl aromatic monomer
contributed units may contain up to 90% of 1,2-microstructure. The
1,2-microstructure can also be constant by maintaining the modifier
to anionic initiator molar ratio at a set value of 0 to 400.
The term 1,2-microstructure as used in the present invention
actually refers to the mode of addition of a growing polymer chain
with a conjugated diene monomer unit. Either 1,2-addition or
1,4-addition can occur. For simplicity, the terms vinyl content or
1,2-microstructure are employed to describe of conjugated
dienes.
The total tapered block copolymer of the present invention can be
represented by the structural formula (I):
wherein n ranges from 5 to 2500. This formula represents the ideal
model of a tapered styrene-butadiene block copolymer formed during
one full monomer charge cycle in the reactor. B.sub.1 /S.sub.1
represents the initial block formed containing 50 to 100% by weight
of styrene (S.sub.1) and 50 to 0% by weight of butadiene (B.sub.1).
The percentage of styrene in each block gradually decreases such
that % S.sub.1 >% S.sub.2. . . >% S.sub.n ; and the block of
B.sub.n /S.sub.n contains from 0 to 30% by weight of styrene and 70
to 100% by weight of butadiene, i.e. % S.sub.n = between 0 to 30%,
% B.sub.n =70 to 100%, and (% B.sub.n +% S.sub.n)=100% of total
monomer feed. The average percent by weight of vinyl aromatic
monomer, preferably styrene, in the tapered copolymer must be
between about 30 to 70%.
In a preferred embodiment, the diene monomer is 1,3-butadiene and
the vinyl aromatic monomer is styrene. The copolymers prepared in
accordance with the invention have molecular weights between 20,000
and 2,500,000, preferably between 75,000 and 500,000 and possess an
average of 30 to 70% by weight of vinyl aromatic monomer
contributed units and 30 to 70% of diene contributed units.
The charge of vinyl aromatic monomer, preferably styrene, into the
reactor can also be maintained at decreasing charged amounts;
thereby producing a tapered block copolymer having a decreasing
styrene content or other vinyl aromatic monomer contributed
content, while optionally tapering the 1,2-microstructure in
accordance with the previously defined procedures. Thus the process
of the present invention can be utilized to prepare tapered
copolymers having: (1) both a tapered vinyl aromatic monomer
content and a tapered 1,2-microstructure per sequential block
formation or tapered vinyl aromatic monomer content and a constant
1,2-microstructure per sequential block formation.
Preferably, the diene monomer, vinyl aromatic monomer and modifier,
if present, and initiator are added via separate feed streams into
the polymerization reactor. The monomer ratios are varied through
the course of the reaction while the modifier level can vary or be
maintained. A rise in modifier level yields an increase in vinyl
content and also aids in encouraging random addition of vinyl
monomer in the tapered blocks of the copolymer. During the course
of the polymerization it will generally be desirable to provide
some form of agitation to the reaction mixture, such as stirring,
shaking, or tumbling. A short stopping agent such as an alcohol may
be employed to terminate the polymerization after the desired
reaction time or at the desired percent conversion of monomers to
copolymer. In general, the conversion of monomers into polymers is
allowed to proceed to about completion. An appropriate antioxidant
can be added at this stage of the process.
The nonaqueous dispersions formed in this polymerization process
have a solids concentration ranging between about 10 to 50% by
weight and are quite fluid. This fluidity permits greatly improved
heat transfer as compared to the fluidity of solutions of SBR
copolymers prepared using solution polymerization techniques. Due
to the relative fluidity of these nonaqueous dispersions, both a
higher molecular weight tapered polymer can be produced and the
concentration of dispersed SBR tapered copolymers in the medium can
be increased by 25 to 100% or more over the maximum allowable
concentrations in solution polymerization techniques.
The elastomeric SBR tapered copolymer can be recovered from the
hydrocarbon solvent by steam desolventization or by drum drying
techniques thus providing energy savings due to higher solids
levels. By proper control of particle size, the polymers can be
recovered by filtration or centrifugation techniques.
The recovered tapered copolymer products, depending on their
molecular weights and compositions, can be used for a variety of
goods such as tires and various rubber molded products and for
adhesive applications.
In the process of the present invention, SBR can be produced in a
low viscosity dispersion having a styrene content as low as 30% by
weight, since SBR's having a styrene content of 30 to 35% by weight
are soluble in technical hexane, the present process enables one to
utilize dispersion polymerization procedures at low styrene levels
previously thought to be polymerizable only in solution. The
present process also permits the production of higher styrene
content tapered polymers in technical hexane without the occurrence
of phase separation.
It is believed that one skilled in the art can, using the preceding
description, utilize the present invention to its fullest extent.
The following preferred specific embodiments are, therefore, to be
construed as merely illustrative of the catalyst system and the
polymerization process of the present invention. All percentages
identified in the examples are by weight unless otherwise
indicated.
EXAMPLE 1
Preparation of "A" Block of Dispersing Agent
A 20 gallon heated jacket reactor was charged with 30.5 lb. hexane.
The reactor was vented and agitated. The reactor was charged with
0.38 cc. of OOPS modifier, 3.2 lb. of 24.65% of 1,3 butadiene in
hexane, and 0.36 lb. of 25.2% of styrene in hexane. The reactor
jacket temperature was set at 200.degree. F. and when the reaction
mix reached 170.degree. F. the
reactor was additionally charged with 36 cc. (24 g) of 3%
n-butyllithium in hexane. The reactor was maintained at
approximately 170.degree. F. for ten minutes, thereby polymerizing
the charged monomers to produce the "A" block of the dispersing
agent.
Preparation of "B" Taper Block of Dispersing
Agent and "C" Tapered Copolymer
The reactor containing the "A" block is charged with 108 cc. (71 g)
of 3% n-butyllithium and thereinafter streams of 24.65% of
1,3-butadiene in hexane at 0.026 lb/min and 25.2% of styrene in
hexane at 0.234 lb/min. Every 20 minutes the butadiene stream was
increased an additional 0.0508 Ib/min and the styrene stream was
correspondingly decreased by 0.0468 Ib/min until a total of 18.3
lb. of the butadiene stream and 14.1 lb. of the styrene stream were
charged. Polymerization was terminated by the addition of
isopropanol and agitation was continued for 20 minutes. A
antioxidant (54 g. BHT) and 100 cc. of water were added and the
polymer dispersion was mixed for 30 minutes, cooled and recovered.
The charge parameters are displayed in Table I. The properties of
the recovered tapered copolymer are displayed in Table II. This
process produced a block taper structure with n=6 in structural
formula (1).
EXAMPLES 2 to 6
The dispersion tapered copolymers of Examples 2 to 6 were prepared
in accordance with the procedure of Example 1 utilizing the
reactant charge parameters displayed in Table I yielding A-B block
dispersing agents and tapered copolymers having properties
displayed in Table II.
EXAMPLE 7
A one gallon reactor was charged with 0.5 lb hexane, 3 g of styrene
in hexane, 27 g of 1,3-butadiene and 0.5 mmole of a dilithium
compound made from a molar reaction 1.1 of one mole of
1,3-diisopropenyl benzene, two moles of s-butyllithium and two
moles of triethylamine (Et.sub.3 N). The mixture was reacted for 15
minutes at 200.degree. F. to produce an "A" block.
Subsequently, 4.5 mmole Di-Li-2Et.sub.3 N was charged into the
reactor, 2.42 lb. of 24.7% solution of a 1,3-butadiene in hexane
blend was charged to a pressurized tank and 1.21 lb of 33% solution
of styrene in hexane and 0.4 lb of hexane was charged to a stirred
tank. The styrene in hexane blend was charged into the reactor over
a 90 minute period. Simultaneously, the butadiene in hexane blend
was charged into the stirred tank containing styrene in hexane over
a 85 minute period. The monomer blend entering the polymerization
reactor at 200.degree. F., was composed of a continuously
decreasing styrene concentration and a continuously increasing
1,3-butadiene concentration, resulting in a steadily changing
composition along the polymer chain.
The reaction was terminated with 5.0 mmole tributyl-SnCl. An
excellent dispersion of low viscosity was obtained.
The recovered polymer has a ML-4 at 100.degree. C. of 51, a Mn of
155,400, a Mw/Mn of 1.58, a 9.5% vinyl content (butadiene =100), a
31.2% styrene content, and a 3.4% block styrene content
(styrene=100).
TABLE I
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Example No. 1 2 3 4 5
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BLOCK "A" CHARGES OOPS (cc) 0.38 0.32 0.32 -- -- 24.65% Butadiene
in Hexane 3.20 3.20 3.20 3.20 3.20 (lb) 25.2% Styrene in Hexane
(lb) 0.36 0.36 0.36 0.36 0.36 3% n-BuLi in Hexane (cc) 36 (24 g) 31
(20 g) 31 (20 g) 9.3* 31 (20 g) "B" BLOCK - "C" COPOLYMER CHARGES
3% n-BuLi in Hexane (cc) 108 (71 g) 93 (61 g) 93 (61 g) 28* 93 (61
g) Polymerization Temp., .degree.F. 212 208 216 214 216 Initial
Metering Rate (lb/min) 24.65% Butadiene/Hexane 0.026 0.100 0.087
0.021 0.021 25.20% Styrene/Hexane 0.234 0.234 0.204 0.186 0.186
Monomer Charge Taper Rate (lb/min) Butadiene 0.050800 0.096000
0.000113 0.000220 0.000230 Styrene -0.046800 -0.021300 -0.001430
-0.000132 -0.000132 Metering Time 122 min. 122 min. 122 min. 123
min. 120 min. Time Increment Between 20 min. 10 min. 5 sec. 5 sec.
5 sec. Monomer Taper Rate Change Total Monomer and Hexane Charge
(lb) Butadiene 18.3 18.3 20.1 21.2 21.2 Styrene 14.1 14.0 12.3 11.2
11.2
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*1 molar DiLi.2Et.sub.3 N
TABLE II ______________________________________ Example No. 1 2 3 4
5 ______________________________________ Mod/Li 0.05 0.05 0.05 0.00
0.00 % `A` Block 10 10 10 10 10 % `A` Initiator 25 25 25 25 25 %
Total Solids 18 18 18 18 18 % Conversion 99.1 99.8 99.5 99.0 98.6
Mooney 40 42 44 56 44 % Styrene 39.3 38.8 34.4 31.2 31.5 % Block
Sty- 29.9 11.3 14.2 7.3 6.2 rene % Vinyl 17.4 17.0 16.4 10.3 8.6 Mn
102756 112027 167984 150401 -- Mw/Mn 1.28 1.23 1.20 1.53 --
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* * * * *